docs: document mounting external storage system

[mdonadoni]

Other than the shared storaged, REANA also supports mounting host directories inside the job containers. This can be achieved by setting the components.reana_workflow_controller.environment.REANA_JOB_HOSTPATH_MOUNTS Helm value.

We should test and document this feature, as it can be useful to mount external storage systems:

1. The external storage system is mounted on every host node (e.g. using DaemonSet)
2. Mount point is mounted inside the container